Quite a number of small motors are used in personal computers and the peripheral devices, IT-related appliances and the like apparatus in the field of advanced technologies. These motors are requested to be more compact in the size, higher in the output and efficiency, along with the ongoing pursuit of compactness and lightness in the appliance sector. Among the total production number of small motors, DC motors account for approximately 70%. Most of the general-use DC motors use rubber bonded ferrite magnets. High-performance DC motors employ ring-shaped bonded magnets produced by compressing a composite of magnetically isotropic Nd—Fe—B system magnetic powder and a rigid thermosetting resin such as epoxy resin. Technological developments in the small motors represent the improvements achieved in the field of bonded magnets manufactured by bonding a magnetic powder with a binder.
FIG. 13 describes the technology combinations used for manufacturing conventional bonded magnets. The magnetic powder can be a hard ferrite system magnetic powder, an Alnico system magnetic powder or a rare earth system magnetic powder. The binder can be a flexible resin (e.g. rubber, thermoplastic elastomer), a rigid thermoplastic resin or a rigid thermosetting resin. The forming process can be a calendering, an extrusion, an injection molding or a compression. Conventional combination of these items is indicated with solid lines.
It has been known that the rare earth magnetic powders can be combined with a flexible resin, a rigid thermoplastic resin and a rigid thermosetting resin. It is also known that the rare earth magnetic powders can be combined with a calendering, an extrusion, an injection molding or a compression. Namely, the rare earth magnetic powders are known to be compatible with any one of the items described in the binders and the processes. In the combination of compression process and rare earth system magnetic powder, however, the binder is limited to a rigid thermosetting resin, such as an epoxy resin.
Meanwhile, there are following disclosures in the flexible bonded magnet sector for use in small motors whose output power is lower than several tens of watts, which being subject of the present invention: Japanese Patent No. 2766746 and Japanese Patent No. 2528574 disclose bonded magnets produced by a process of rolling magnetic materials made of rare earth element system magnetic powders and flexible resins. Permanent magnet type motors using the above sheet-formed flexible bonded magnets are also disclosed. However, the maximum energy product (hereinafter referred to as MEP) of these bonded magnets is in the level as low as 50 kJ/m3. Japanese Patent Laid-Open Application No. H5-299221 teaches a method of manufacturing a bonded magnet by rolling a mixture of rare earth-iron-nitrogen system magnetic powder and flexible resin. MEP of this bonded magnet is 42 kJ/m3. Japanese Patent Examined Publication No. H6-87634 discloses a bonded magnet manufactured by compressing a magnetically isotropic R—Fe—B (R signifying at least either one of Nd and Pr) rare earth system magnetic powder and a rigid epoxy resin. A permanent magnet type motor using a multipole-magnetized ring magnet is also disclosed. MEP of which is 77 kJ/m3.
In order to implement a bonded magnet for small motors that is more efficient than the above-described conventional ones, it is essential to have a new binder system and an optimized processing. Specific characteristics needed for the new binder system are that it has a strong adhesive strength and the density of bonded magnet can be raised easily. Insufficient adhesive strength causes partial fracture of magnet body, and the fractured particle would scatter inviting serious damages. If the density is not high enough, the magnetic characteristics do not improve. The present invention aims to solve the above problems, and offer a method for manufacturing a highly-efficient and reliable bonded magnet for use in small motors. A method for manufacturing permanent magnet type motors using the bonded magnet is also disclosed in the present invention.